The present invention relates to an electrochemical process for obtaining alkali metals from aqueous solution by an electrochemical process, and also to an electrolysis cell for carrying out this process. The invention further relates to an electrochemical process for recycling alkali metals from aqueous solution.
For the purposes of the present invention, alkali metals are lithium, sodium and potassium.
Lithium is important as a basis for inorganic chemistry and is used in a variety of applications, such as lithium batteries, organolithium compound preparation, and addition to aluminum or to magnesium to give alloys. Lithium is prepared industrially by melt electrolysis of a eutectic mixture of lithium chloride and potassium chloride at from 400 to 460xc2x0 C. (Ullmann""s Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release). This process has high energy consumption (28-32 kWh/kg of Li). The process also has the serious disadvantage that only anhydrous lithium chloride can be used. The lithium chloride, which is mainly available as aqueous solution, therefore has to be treated by a high-energy process to give the anhydrous solid. Since lithium chloride is hygroscopic, its drying and handling are particularly expensive.
Organic reactions with lithium frequently produce aqueous lithium salt solutions. The increasing use of lithium batteries also produces lithium-containing waste, and this, too, can be converted into aqueous lithium solutions. Since lithium is very expensive, even in the form of its salts, recycling of lithium is of interest.
U.S. Pat. No. 4,156,635 and J. F. Cooper et al., Proc. Electrochem. So. 1995, pp. 95-11, 280-290, describe a process for the electrochemical preparation of lithium from an aqueous lithium salt solution by using a lithium amalgam electrode. For this, a lithium solution, in particular a lithium hydroxide solution, is electrolyzed using an amalgam cathode. This forms lithium amalgam, which becomes the anode in a second electrolysis cell. Lithium cathode and amalgam anode here are separated with the aid of boron nitride seals. The electrolyte in this second electrolysis cell is a 2 cm salt melt of two alkali metal iodides (preferably LiI and CsI or, respectively, LiI and KI), while lithium metal is deposited at the cathode. The current density here is from 1 to 4 kA/m2 without mass transfer limitation. In the recovery of lithium from the amalgam in this process the current yield achieved is only from 81 to 87%. A particularly serious problem is that the lithium obtained has mercury contamination, since the mercury can diffuse through the electrolyte.
Sodium is important as a basis for inorganic chemistry and is used for preparing sodium amide, sodium alcoholates and sodium borohydride, for example. It is obtained industrially by the Downs process, by electrolyzing molten sodium chloride. This process has high energy consumption of xe2x89xa710 kWh/kg of sodium (Bxc3xcchner et al., Industrielle Anorganische Chemie, 2nd edition, Verlag Chemie, pp. 228 et seq.). The process also has the serious disadvantage that the electrolysis cells are damaged by solidification of the salt melt on shutdown. The sodium metal obtained by the Downs process also has the disadvantage of calcium contamination caused by the process, and although subsequent purification reduces the residual content of calcium it never removes it entirely.
Potassium is also important as a basis for inorganic chemistry and is used for preparing potassium alcoholates, potassium amides and potassium alloys, for example. Nowadays it is primarily obtained industrially by reacting potassium chloride with sodium. This first gives NaK, which is then fractionated by distillation. A good yield is obtained by constantly drawing off potassium vapor from the reaction zone, thus shifting the reaction equilibrium toward the potassium side (Ullmann""s Encyclopedia of Industrial Chemistry, 6th edition 1998, Electronic Release). A disadvantage is that the process operates at high temperatures (870xc2x0 C.). In addition, the resultant potassium comprises about 1% of sodium contaminant and therefore still requires purification by a further rectification. The greatest disadvantage is that the sodium used is expensive. The reason for this is that sodium is obtained industrially by the Downs process, by electrolyzing molten sodium chloride, and the energy usage necessary here is at least 10 kWh/kg of sodium, corresponding to about 5.3 kWh/kg of potassium (for 100% yield).
GB 1,155,927 describes a process which uses a solid sodium ion conductor, e.g. xcex2-Al2O3, with amalgam as anode and sodium as cathode, to obtain sodium metal electrochemically from sodium amalgam. However, the process described in GB 1,155,927 does not give the results described there with regard to sodium conversion, product purity and current density. The system described there, furthermore, develops instability over the course of a few days when the claimed temperature range is adhered to. Electrolysis cells which are used to prepare alkali metal by an electrochemical process and which have a solid ion conductor are frequently unsuitable for permanent operation over a long period. One reason for this is that after a certain period of operation the solid ion conductor becomes mechanically unstable.
It is an object of the present invention, therefore, to develop a process which does not have the disadvantages described above (high energy consumption, calcium content in the sodium, high temperature, etc.). A further object is to provide an electrolysis cell suitable for carrying out this process. A further object is to find a process which enables recycling of alkali metals from aqueous alkali metal waste, in particular lithium from aqueous lithium waste.
We have found that this object is achieved by means of a process which comprises carrying out an electrolysis in a novel electrolysis cell with a solid ion conductor. The ion conductor here separates the electrolysis cell into two parts. In one part is the liquid alkali metal which serves as cathode. In the other part, and in contact with an anode, is an aqueous solution of a salt of this same alkali metal. Any desired commercially available anode materials can be used as anode.
Alkali metal ion conductors of this type are frequently not resistant to water and/or to alkali metals, and the experiment therefore leads to damage of the alkali metal ion conductors after only a short period. This damage can comprise either mechanical failure of the ion conductor or loss of its conductivity. A further aim of the invention is therefore to keep the ion conductors stable over a prolonged working life. The working life of the ion conductors can be increased markedly by applying an ion-conducting protective layer to the appropriate side of the ion conductor.
The present invention therefore also provides an electrolysis cell comprising an anode compartment which comprises an aqueous solution of at least one alkali metal salt, a cathode compartment and a solid electrolyte which separates the anode compartment and the cathode compartment from one another, wherein that part of the surface of the solid electrolyte which is in contact with the anode compartment and/or that part of the surface of the solid electrolyte which is in contact with the cathode compartment has/have at least one further ion-conducting layer.
The present invention also provides a process for preparing an alkali metal from an aqueous solution comprising at least one salt of this alkali metal, using this electrolysis cell.
There are generally no restrictions in relation to the aqueous alkali metal salt solution in the anode compartment, and any suitable alkali metal salt solution may be used. Besides pure aqueous solutions, it is also possible to use mixtures with water-miscible organic solvents, as long as the organic solvents are stable under the reaction conditions. Examples of solvents of this type are alcohols, e.g. methanol and ethanol, and also carbonates, e.g. propylene or ethylene carbonate.
There are likewise no restrictions in relation to the shape and design of the anode compartment, as long as it is certain that the walls of the anode space are resistant to the anode solution and that that part of the surface of the solid electrolyte which is in contact with the anode compartment is sufficient for the process to be carried out to prepare the alkali metal.